Neogene paleoelevation of intermontane basins in a narrow, compressional mountain range, southern Central Andes of Argentina
Introduction
Constraints on the elevation history of mountain ranges provide topographic boundary conditions that are important for evaluating tectonic (DeCelles et al., 2009, Hilley et al., 2004, Isacks, 1988, Jordan et al., 1983), geodynamic (Meade and Conrad, 2008, Whipple and Meade, 2006, Willett, 1999) and paleoclimate models (Jeffery et al., 2012). Over the past decade, large orogenic plateaus have been the major focus of paleoaltimetry studies (e.g., Garzione et al., 2000, Garzione et al., 2008, Mix et al., 2011, Rowley and Currie, 2006) in an attempt to couple deformation histories and geodynamic processes with elevation history (Ehlers and Poulsen, 2009, Hoke and Garzione, 2008, Lamb, 2011, Mulch et al., 2006). However, elongate, longitudinally oriented mountain ranges that are located at right angles or obliquely with respect to the prevailing wind directions and atmospheric moisture transport are especially attractive targets to assess the regional impact of topographic growth on atmospheric circulation patterns and ensuing changes in climate and surface processes. Yet, narrow, non-collisional mountain ranges dominated by fold-and-thrust belts, which should have simpler uplift and sedimentary histories, have received less attention (Blisniuk and Stern, 2005, Chamberlain et al., 1999, Hren et al., 2010). Measuring more than 7000 km in length and at the interception of two major precipitation regimes sourced in the southwest and northeast, respectively, the meridionally oriented Andes are a prime site to examine these relationships (Bookhagen and Strecker, 2008, Garreaud et al., 2009, Montgomery et al., 2001, Strecker et al., 2007).
Ideally, paleoaltimetry studies span sites likely to have remained at similar elevations through time as well as sites thought to have undergone significant elevation change over the same period. The eastern flanks of the southern central Andes in central Argentina between 30°S and 35°S (Fig. 1) contain a combination of largely synchronous late Cenozoic intermontane and foreland basin deposits rich in calcareous paleosols (Figs. 2A and 2B), making it an excellent locality for constraining elevation history in the context of crustal shortening (Fig. 2C). Furthermore, the magnitude of deformation at these latitudes is well-constrained (Allmendinger et al., 1990, Cristallini and Ramos, 2000, Giambiagi et al., 2012) with total shortening decreasing nearly threefold from north to south across the study area (Fig. 2C) and geophysical data indicating a 15 km decrease in crustal thickness over the same interval (Gans et al., 2011, Giambiagi et al., 2012). Despite these dramatic changes in total crustal shortening and differences in geophysical characteristics, there is no significant change in mean elevation (Fig. 2D). This study uses the isotopic composition of pedogenic carbonates preserved in topographically distinct Miocene to Pliocene sedimentary basins to evaluate the spatial and temporal relationships between crustal thickening and topographic growth over a 500 km segment of the Andes with the goal of understanding what drives elevation change in narrow, linear mountain ranges.
Section snippets
Geologic and tectonic background
The western margin of the southern South American continent has been convergent since at least the Jurassic, if not earlier (Mpodozis and Ramos, 1989). An abrupt transition in the subduction angle of the Nazca Plate occurs at ∼33°S (Fig. 1), with the area to the north constituting the wider section of flat-slab subduction (Fig. 1; Anderson et al., 2007, Cahill and Isacks, 1992). South of 33°S, dip of the subducting slab returns to ∼30° (Cahill and Isacks, 1992). The initiation of flat-slab
Sampling
We collected pedogenic and other authigenic carbonates exposed in intermontane and foreland basin successions on the eastern flanks of the Andes (Fig. 3). The stable isotope composition of authigenic carbonate formed in soils records information about the types or productivity of vegetation, the isotopic composition of meteoric waters at the time the soil formed (e.g., Cerling and Quade, 1993), and may be used to reconstruct past elevations (e.g., Rowley and Garzione, 2007). Pedogenic carbonate
Thin sections
A total of 13 thin sections from the Cacheuta Basin were examined for evidence of recrystallization or strong diagenetic overprinting; 12 show primary micritic textures (supplementary Fig. 1b) with very minor spary calcite occasionally filling dessication cracks (Supplementary material Fig. 3c). One thin section, from a calcrete horizon at the base of the Mariño Formation, shows the primary texture of radial crystals around rhizoliths (supplementary Fig. 1b; Klappa, 1980).
Geochronology
The analysis of 6
Discussion
Here, we present Miocene stable isotope data between 35°S and 32°S. Given this large latitudinal range, our data have the potential to elucidate topographic and paleoenvironmental change within an important transition in the major atmospheric circulation systems impacting the southern hemisphere (e.g., Haselton et al., 2002). Our strategy of sampling pedogenic carbonates from a variety of elevations and tectonic positions, as well as from sections reflecting different episodes and styles of
Conclusions
We used the δ18O values of pedogenic carbonate from foreland and intermontane basin sediments presevered in different tectonic positions in the southern Central Andes of central Argentina. By designing our sampling to include sites that have always been east of the modern range front, we are able to control for regional variations in non-elevation related factors such as climate or environmental change. No major, coherent isotopic shifts are observed in our isotope records, which span up to
Acknowledgments
This project was supported by NSF grant OISE-061957 and Alexander von Humboldt Foundation fellowships for Hoke to be in residence at the Universität Potsdam. Strecker acknowledges funding from the Leibniz Award of the Deutsche Forschungsgemeinschaft (DFG) project STR 373/18-1, and Garzione acknowledges funding from NSF grant EAR-0635678. The authors thank Florencia Bechis, José Mescua, Julieta Nobile, Elena Previtera, Miguel Ramos, José Rosario and Gisela Reyna for assistance in various field
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